The present invention pertains to a method for processing magnetotelluric survey data and more particularly to a method for removing the effects of near surface anomalies on resistivity curves produced through magnetotelluric surveys.
Several tools for geological exploration research of the earth's subsurface formations exist in present technology. Although the most widely accepted tool is seismic surveying, other tools such as magnetotelluric surveys may be used in certain instances. The magnetotelluric survey is usually less expensive than a seismic survey. However, in general, a magnetotelluric survey is not as accurate nor does it possess as high a degree of resolution as a seismic survey.
A method for taking magnetotelluric measurements is to place four electrodes in a pattern defining a square with each electrode in a corner. Of the four electrodes, opposite corners of the square are paired, thus the lines defined by connecting paired electrodes are perpendicular to each other. Electrical impulses are received from natural electrical energy in the earth from one electrode to the other electrode of the pair. For complete magnetotelluric survey information magnetometer coils are needed to measure the magnetic effects of the earth at the location being surveyed. The electrode pairs give the E components of the survey, normally designated as E.sub.x and E.sub.y. The E measurements are correlated with H measurements (H.sub.x and H.sub.y) which are detected by the magnetometer coils. The frequency of the electrical waves indicates the depth of the formation for which the resistance is measured. For example, the resistance of a shallow subsurface formation can be measured by detecting high frequency telluric electromagnetic waves. To obtain the resistance of deeper formations, lower frequencies of the telluric electromagnetic waves are measured. The depth of the formation having the measured resistivity is calculated by combining the telluric electrical field and the magnetic field measured. Resolution of the exact point for which the resistivity is being measured at a given depth deteriorates as a greater depth is surveyed. For a more detailed discussion of magnetotelluric surveying and electrode placement techniques, reference is made to U.S. Pat. No. 4,286,218, titled "Multiple Site Magnetotelluric Measurements" filed by Marvin G. Bloomquist, et al, assigned to the same assignee as the present application.
In a magnetotelluric survey, the resistivity of the subsurface formations beneath the measuring device is frequently the information displayed. This information may be plotted illustrating resistivity as a function of frequency such as the graphical representation of FIG. 1. As illustrated in FIG. 1A, by convention two separate resistivity curves (R.sub.x and R.sub.y) are routinely generated during the processing of magnetotelluric data. These two curves are assumed to be "parallel to electrical strike" and "perpendicular to electrical strike" respectively. From the two curves labelled R.sub.x and R.sub.y, approximate true resistivity versus depth curves are calculated in an attempt to generate a highly smooth resistivity distribution of the subsurface. However, due to surface or near surface anomalies, the R.sub.x and R.sub.y curves will separate or split into R'.sub.x and R'.sub.y curves and be parallel to each other. Thus a DC like bias is present in the data and illustrated in FIG. 1B. This DC bias is normally attributed to localized surface anomalies such as "pipelines, geological faults, lithology variations, small caverns", etc. A subsurface formation which dips will produce different readings depending on the relationship between the configuration of the measuring electrode pair and the angle of the formation dip.